Modern day wireless communication devices, such as mobile phones, require at least one clock operating at a stable reference frequency to function correctly. A stable reference frequency is typically obtained by using a free-running crystal oscillator as a reference clock. However, modern mobile terminals have strict frequency stability requirements which cannot be achieved using only a free-running crystal oscillator due to limiting factors such as temperature dependencies, supply voltage variations, and aging effects, which may cause frequency variations and/or frequency drift.
To achieve a more stable reference frequency an automatic frequency control (AFC) mechanism can be utilized. The AFC estimates a frequency error on the received signal in relation to the operating frequency of the mobile terminal and applies a correction to the free-running reference clock in order to synchronize the frequency of the mobile terminal to the counterpart transmitting the received signal. In this way frequency variations and frequency drift may be kept to a minimum.
The current trend in mobile communication is to increase the functionality of the mobile terminal by integrating functions such as Global Positioning System (GPS), Bluetooth communication, FM-radio, etc. into the mobile terminal. These integrated functions are often, due to cost-, design-, or other aspects, clocked by the same AFC corrected reference clock as the mobile terminal transceiver. However, sharing an AFC corrected reference clock may cause problems.
The frequency changes that occur when an AFC correction is applied to the reference clock will be passed on to all circuits sharing the same reference clock. These changes, even if they are relatively small, may disrupt or degrade the performance of some of the functions that require a continuous and stable reference clock. Thus, there is a need for improving the method for generating and distributing reference clock signals in a modern mobile terminal.